In a mine of a non-ferrous metal and a plant from which waste water containing heavy metal ions is generated such as a smelter, a treatment for separating heavy metal ions as an effluent sediment from an effluent so that the heavy metal ions are not contained in a discharged effluent is conducted by providing a step of an effluent treatment.
For example, as the method for separating a heavy metal from waste water, a method including adding an alkali such as lime stone or lime hydrate to an effluent to adjust the pH of the effluent to form a slurry to thereby precipitate and fix the heavy metal as a sediment is generally performed.
Among non-ferrous metals, ores of low-grade nickel oxide ores such as lateritic ore tend to contain a relatively large amount of manganese, which is a heavy metal. In nickel smelting using a high pressure acid leach process (HPAL process), which has been frequently used in recent years, manganese remains in an effluent from which nickel has been recovered. Therefore, a method including adding an alkali to an effluent to separate manganese as a neutralized sediment is adopted so that effects that the manganese is directly discharged to thereby color rivers, and the like would not be generated.
However, in such method, there was a problem that other metal ions that are not necessary to be precipitated and separated such as magnesium also form a sediment at the same time, and thus the amount of the sediment increases. When a sediment is formed by the metal ions that are not necessary to be precipitated and separated, unpreferable effects that the use amount and cost of an alkali required for neutralization increase, the capacity of a place for disposal is pressed, and the like are caused.
Therefore, a method for selectively precipitating and separating only manganese without co-precipitating magnesium, for which precipitation is unnecessary, has been demanded.
For example, in Patent Literature 1, a method for selectively precipitating and separating only manganese without co-precipitating magnesium, by a so-called oxidation-neutralization process, which includes adjusting the pH of waste water to be in the range from 8.2 to 8.8, and blowing an oxygen gas into the waste water to maintain the redox potential to be in the range from 50 to 300 mV, is reported.
However, also in this method, magnesium and aluminum were precipitated at the same time in the case when aluminum ions were contained in the liquid, and thus problems that the neutralizer is required in an amount for neutralizing the magnesium in addition to an amount for removing the manganese, and that the amount of generation of a sediment is increased, have not been sufficiently solved.
Therefore, Patent Literature 2 indicates a method for separating aluminum including adjusting the pH of waste water to be in the range of from 4 to 6 in advance of an oxidation-neutralization treatment of the effluent. The co-precipitation of magnesium and aluminum is prevented by using this method, and thus it becomes possible to effectively separate aluminum.
However, there was a problem that the aluminum sediment formed by the method of Patent Literature 2 is a very bulky sediment and thus is voluminous in handling. This is because the particles of the aluminum sediment are fine and the moisture content in the sediment is high, and thus a considerable part of the magnesium in the effluent tends to be contained as adhesive moisture. Furthermore, since the specific gravity of the aluminum sediment is light as about 1.05 to 1.10 g/ml, which is substantially the same as that of the effluent, the sediment is difficult to settle out. In addition, there are many inconvenient characteristics such as that honeycomb airspaces are easily formed in the sediment by incorporation of air during stirring. Especially, these tendencies were significant in the case of waste water with a concentration of aluminum ions of greater than 2 g/l.
Furthermore, a thickener is frequently used in industrially performing solid-liquid separation. In an operation using a thickener, a flocculant is added, and the solid content in a slurry fed to the thickener settles out on the bottom part of the thickener and becomes a sediment called an underflow, and is suitably removed from the thickener. Since a quantity of liquid adheres to the removed underflow, it is necessary to treat the dehydrated sediment by performing fine filtration by using a filter press or the like.
At this time, an underflow in which the liquid adhered to the underflow is small, i.e., a high-concentration underflow is preferable since the size of the filtration equipment such as the thickener or filter press can be saved and the neutralizer that is added for the neutralization treatment of the adhered liquid after the separation can also be saved. However, especially in the case when a honeycomb sediment as mentioned above is formed, the substance amount of the underflow required for fine filtration is increased, and thus there are problems in costs such as increase in the capacity of the equipment in accordance with the increase in the substance amount of the underflow.
As a method for solid-liquid separation of a poorly settleable and poorly filterable slurry, for example, the method indicated in Patent Literature 3 is exemplified. The method described in Patent Literature 3 is a method for treating a poorly settleable and poorly filterable slurry containing colloidal silica, and a method for improving the settleability and filterability is suggested. Specifically, the settleability and filterability are improved by adding bentonite to a slurry containing colloidal silica.
However, this method described in Patent Literature 3 has little effect on components other than colloidal silica such as magnesium. Furthermore, since bentonite is newly added from outside, the increase in the amount of the sediment by the addition of bentonite is not negligible, and thus there is a limit to apply this method.
As mentioned above, a suitable method by which a high-density aluminum sediment and an effluent from which aluminum has been removed can be obtained from waste water that contains aluminum besides manganese and magnesium as in an effluent discharged in the smelting of a nickel oxide ore has not been developed.